Lens for radio-frequency waves



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LENS Fox RADIO FnEQUENcY'wAvEs frequency lens disposed betweensurfaces;` and Fig. 10 is a sectional view of Patented Feb. 4, 1947STATES i 2,415,352 Y Lens Foa RADIO-FREQUENCY WAVES Harley A. Iams,Princeton,

N. J., assigner to Radio Corporation of America, a corporation ofDelaware 16 Claims.

'I'his invention relates to lenses for radio frequency waves and moreparticularly to such lenses in which the wave reflections from thesurfaces are reduced.

It is known that radio frequency waves may be reflected and refracted.Reflections or refractions of radio waves occur when a wave travellingthrough a medium'reaches a, discontinuity in the medium or goes from onemedium into another in which its velocity is different. If the surfaceor object in the path of the wave is composed of non-conductivematerial, both reflection and transmission with refraction may occurdepending upon the angle between the direction of propagation and thereflecting surface, and upon other factors which do not requirediscussion here.

Lenses for focusing radio frequency waves have been made of materialssuch as pitch, paran-ln, and other insulators. The abrupt change fromthe dielectric constant of the medium of propagation to the dielectricconstant of the lens produces an undesired reflection of the waves. Theinstant invention relates to the reduction of such undesiredreflections.

One of the objects of the invention is to provide means for reducing theundesired reflection from the surface of a radio frequency lens. Anotherobject is to provide an improved lens for radio waves of ultra highfrequency. Another object is to provide a lens including reflectionreducing surfaces whereby radio waves will be transmitted with lessloss. An additional object is to provide an improved lens with askeletonized surface for minimizing the reflection of radio frequencywaves. l

The invention will be described by referring to the accompanying drawingin which Figs. 1 and 2 y are plan and section views, respectively, ofone embodiment of theinvention; Figs. 3 and 4 are plan and sectionviews, respectively, of a radio conductive members; Figs. 5 and 6 areplan and section views, respectively, of a biconvex lens includingreflection reducing coatings; Figs. 7, 8 and 9 are plan, section viewsand a detailed section view, respectively, of a biconvex lens includinga skeletonized a modification of the invention.

Referring to Figs. 1 and-2, a lens including a flat core I of waverefracting material such as Micalex is constructed with one curvedsurface 3 and one plane surface 5. By Way of example, the dimensions areshown for a lens having a focal length of 1'71/2 inches (measured fromthe straight edge) which was used to form a line image of a sceneilluminated with radiation having a frequency of 24,000 megacycles. Thecore material has a dielectric constant of 6.1. Coatings '7 and 9,consisting of Lucite or polystyrene having a thickness 0f approximatelyone quarter wave length in the coating material, are cemented orotherwise secured to the effective surfaces 3, 5 of the lens. Thedielectric constant of the coating material should be the square root ofthat of the core material; since the dielectric constant of polystyreneis 2.5, it is well suited for coating MicaleX. .In the case of thedescribed lens, a substantial increase in transmission and a substantialreduction in undesired reflections were observed when radio waves of afrequency of 24,000 megacycles were applied. The image appears in thefocal line represented by the dash line I I.

The lens described in the foregoing paragraph was inserted between apair of conductive sheets I3, I5, preferably spaced by 1/2 wavelength orless. The sheets may be curved to form an aperture Il which may bedisposed otherwise than in geometric alignmentwith the effective surface'I of the lens. The conductive sheets guide the radio waves toward thelens and from the lens to the focal line II. The lens may be used fortransmission by applying the wave energy at the open end of the sheetswhich may be shaped or bent to conform to the focal line.

In place of the section of plano-convex lens of Figs. 1 and 2, theinvention may be applied to a biconvex lens as illustrated in Figs. 5and 6 or '7 and 8. By way of example, a lens I9 was formed of Micalex.The following dimensions are given by way of example rather thanlimitation: diameter 14 inches, radius of each face 40 inches, and focallength 13 inches. The coatings 2| and 23 should be made of an insulatorhaving a dielectric constant of the order of 2.5 and a thicknesssubstantially equal to a quarter wave length of the applied radiofrequency waves as measured in the insulator. Since this thickness ofthe reflection reducing coatings is based upon normal incidence, itfollows that the coating will not minimize rreflections at all angles.For incidence other than normal, the ideal thickness is somewhatgreater, being cos 0 where E is the dielectric constant of the material.In the case of Micalex, E=6.1; therefore the material to be removedequals 29% of the total surface layer. In the case of polystyrene, E=2.5and the material to be removed equals 39% of the surface layer.

Another method of reducing the reection is to avoid abrupt changes indielectric constants.

In a lens such as illustrated in Figs. 1, 2, 3 and 4.

the reflection may be reduced by tapering the effective surface as shownin Fig. 10. The ideal length t1 of the taper may -be determined from theequation where A2=wave length in air, 7q=wave length in material oflens, p.=index of refraction of material, and a=any integer. Forpolystyrene and radio Waves having a length of approximately 1/2 inch inair, the reiiection was found to fall steadily as d was increased fromzero to about o inch, and to be negligible for any length over W inch.

In a lens as illustrated in Figs. 7 and 8, reection may also be reducedby the use of tapered grooves, instead of the holes as shown. In thiscase, there should preferably be two or more grooves per wave length,and the depth should be as indicated in the preceding paragraph.

Thus the invention has been described as an` improved lens for radiofrequency waves. The refiections are reduced by applying a coating tothe lens surface, or by skeletonizing the surface layer. The undesiredreflections may also be reduced by tapering the eifective surfaces ofthe lens, i. e., by tapering the surfaces which reiiect the radiowavesinto the medium from which the waves have come.

I claim as my invention:

l. An ultra high frequency device consisting of an electric lens of awave refracting material and l a reflection reducing surface disposed onat least one effective surface of said lens.

2. An ultra high frequency device consisting of an electric lens of awave refracting material having substantially parallel sides and areneetion reducing surface disposed on at least one effective surface ofsaid lens.

3. An ultra high frequency device consisting of an electric lens of aWave refracting material and a reiiection reducing coating disposed onat least one eiective surface of said lens.

4. An ultra high frequency device consisting of an electric lens of awave refracting material having substantially parallel sides and areliection reducing coating disposed on at least one effective surfaceof said lens.

5. An ultra high frequency device consisting of an electric lens of awave refracting material and a surface having an effective thickness ofan odd number of quarter wave lengths of said waves. disposed on atleast one wave impinging surface of said lens whereby the reflection ofsaid waves from said surfaces is reduced.

6. An ultra high frequency device consisting of an electric lens of awave refracting material and a coating having an effective thickness ofan odd number of quarter wave lengths of said waves in said coating,disposed on at least one wave impinging surface of s aid lens wherebythe reection of said waves from said surfaces is reduced.

7. An ultra high frequency device consisting of an electric lens of awave refracting material having at least one curved surface and askeletonized layer disposed on said curved surface and having aneifective thickness of an odd integral number of quarter wave lengths ofthe applied electric waves whereby the reection of said waves from saidsurface is reduced.

8. An ultra high frequency wave focusing device consisting of anelectric lens of a wave refracting material having substantiallyparallel sides and at least one curved surface and a pair of conductiveelements disposed parallel to each other, parallel to said surfaces andextending beyond said lens and away from said curved surface..

9. A device of the character of claim 8 in which the pairs of conductiveelements are curved in the region extending away from said curvedsurface.

10. A device of the character of claim 8 in which a reflection reducingcoating is disposed on said curved surface.

11. An ultra high frequency device consisting of an electric lens of awave refracting material having at least one curved surface and havingparallel sides and means for reducing the index of refraction of thematerial on said curved surface and having an effective thickness of anodd number of quarter wave lengths of the applied electric waves wherebythe reflection of said Waves from said surface is reduced.

12. An ultra high frequency wave focusing device consisting of anelectric lens of a wave refracting material having substantiallyparallel sides and at least one curved surface and a pair of conductiveelements disposed-parallel to each other, parallel to said surfaces andextending beyond said lens in at least one direction.

13. An ultra high frequency wave refracting device consisting of waverefracting material having substantially parallel sides ve or more wavelengths long perpendicular to the direction of motion of the wavefrontplaced between parallel conducting sheets separated by 7i/2 or less.

14. An ultra high frequency device consisting of on electric lens of awave refracting material and a reiiection reducing surface, saidsurfaceincluding at least one projecting portion tapering along the axis of thelens.

15. An ultra high frequency device consisting of an electric lens of awave refracting material and a reiiection reducing surface, said surfaceincluding at least one projecting portion -tapering along the axis ofthe lens in accordance with the equation in which d=the length of' thetaper, 7i1=wave length in material of lens, M=wave length in air,

0f refl'aCiOn Of material, and G=ny REFERENCES CITED 16. radio wavedevice consisting of a mate- The following references are of record inthe rial having a. characteristic dened by the equame 0f this Patent:tion n=\/e' where n=ndex of refraction and 5 UNITED STATES PATENTSe=delectric constantl and a surface layer having an eiective depthsubstantially equal to an odd Ilggeos Zwolgyag Jungjzaelgm integralnumber of quarter wave lengths of the 2273447 om Feb 17' 1942 appliedWaves and in which e'=n" less than 4 10 2'361'589Berufen-tee-{5-:2::0:11:31'1944` at the frequency of the applied waves.2:3311716 Nadeauet al.' oct. 12: 1944 HARLEY A, IAMs. 2,240,157 Gagnon,et a1. Apr. 2,9, 1941

